Vapor compression systems, such as heat pumps, refrigeration and air-conditioning systems, are widely used in industrial and residential applications. The introduction of adjustable actuators such as variable speed compressors, variable position valves, and variable speed fans to the vapor compression cycle has greatly improved the flexibility of the operation of such systems. It is possible to use these new components to improve the efficiency of vapor compression systems by controlling the components correctly.
An operation cycle of the vapor compression system starts by compressing the refrigerant to a high-temperature, high-pressure vapor state, after which the refrigerant flows into a condenser. Because the air flowing over the condenser coils is cooler than the refrigerant, the refrigerant cools to form a high-pressure, low-temperature liquid when exiting the condenser.
Then, the refrigerant passes through a throttling valve that decreases the pressure. The low-pressure refrigerant boils at a lower temperature, so the air passing over the evaporator coils heats the refrigerant. Thus, the air is cooled down, and the low-pressure liquid refrigerant is converted to a low-pressure vapor. This low-pressure, low-temperature vapor then enters the compressor, and the operation of the vapor compression system is continues to cycle.
The operation of the typical vapor compression system is affected by a set of environmental parameters, such as thermal load on the system as well as air temperatures at an evaporator and a condenser. Some of these environmental parameters, such as the indoor temperature, have a desired value, i.e., a setpoint, for users of the vapor compression system. For example, the setpoint can be one variable, e.g., the indoor temperature. Also, the setpoint can be a set of multiple variables, such as the temperature and relative humidity of the indoor air.
The operation of the vapor compression system is also defined by a set of thermodynamic parameters of the refrigerant, such as an evaporating pressure, the amount of superheat at the evaporator outlet, condensing pressure, and the amount of cooling at the condenser outlet. The setpoint can be provided for both the environmental and the thermodynamic parameters.
FIG. 1 shows a conventional vapor compression system 100 including an evaporator fan 114, a condenser fan 113, an expansion valve 111, and a compressor 112. The system can be controlled by a controller 120 for accepting setpoints 115, e.g., from a thermostat, and inputs from a sensor 125, and outputting a set of control signals for controlling operation of the components. The controller 120 is operatively connected to a set of control devices for transforming the set of control signals to a set of specific control inputs for corresponding components. For example, the controller is connected to a compressor control device 122, to an expansion valve control device 121, to an evaporator fan control device 124, and to a condenser fan control device 123.
The controller controls operation of the vapor compression system such that the setpoint values are achieved for a given heat load. For example, a speed of the compressor 112 can be adjusted to modulate a flow rate of a refrigerant. The speed of the evaporator fan 114 and the condenser fan 113 can be varied to alter heat transfer coefficients between air and heat exchangers. The change in the expansion valve 111 opening can directly influence a pressure drop between the high-pressure and the low-pressure in the vapor compression system, which, in turn, affects the flow rate of the refrigerant, as well as superheat at the corresponding evaporator outlet.
In most vapor compression systems, the controller 120 issues commands to various control devices such as the condenser fan control device 123, the compressor control device 122, the expansion valve control device 121, or the evaporator fan control device 124. The objective of the controller is to cause the vapor compression system to track the setpoints 115 such as a desired room temperature or superheat temperature. The controller uses information from sensors 125.
Vapor compression systems are known to consume large quantities of energy, and therefore are costly to operate. Accordingly, it is desired to determine the set of control inputs that optimizes a performance of the vapor compression system. A number of methods for controlling operations of the vapor compression system are known.
For example, one method determines optimal energy consumption by comparison of operation of the vapor compression system controlled by modulating the speed of the condenser fan. However, large changes in the cooling load might result in suboptimal energy consumption because the system does not apply sufficient adjustments to the speed to determine the optimal value of the speed.
Another method for controlling a vapor compression system considers the possibility of sudden change in the environmental or thermal load requirements, monitors the vapor compression system in real-time, and determines, based on these real-time measurements, a set of parameters to enable the system to operate at maximum coefficient of performance. However, that method is time consuming, and requires substantial real time computational resources.
Another method first determines amount of heat flow across an evaporator or a condenser. Next, the amount of heat flow is used to determine the set of optimal control inputs. As the amount of heat flow is directly related to the operation of the vapor compression system, the determination of the heat flow is difficult to avoid. However, there are applications in which it is desired to determine the optimal set of control inputs without determining the amount of heat that the vapor compression system needs to transfer in accordance with a desired setpoint.
Yet another method reduces energy consumption of cold water or hot-water in the air conditioner by measuring the room temperature, and retrieving a value of a valve opening from a valve opening table using the room temperature as an index. However, conventional vapor compression systems typically have number of different components, including but not limited to the valve, which need to be controlled concurrently. Moreover, that method determines the valve opening based on an outside environment conditions only, which are not always optimal.
Accordingly, there is a need for a control system and a method for controlling operation of the vapor compression system such that heat load of the operation is met and a performance of the system is optimized.
Also, vapor compression systems are widely deployed in many environments. Therefore it is desirable to adapt the control system to maintain optimal efficiency of the vapor compression system for various thermodynamic environments and conditions.
Furthermore, it is well known that the characteristics of the vapor compression system vary over time. The refrigerant can slowly leak out of the system, and accumulation of debris on the heat exchangers can significantly alter the heat transfer. Therefore, it is also desirable to provide such control system that optimizes the performance of the system adaptively in consideration of those changes in system behavior.